Prior art conductive pressure-sensitive adhesives are compositions comprising an adhesive component such as an acrylic resin, a blend of acrylic resin with rosin, natural rubber or butyl rubber, in admixture with a conductive powder such as copper powder. Since these conductive pressure-sensitive adhesives are crosslinkable in a relatively low temperature range below 100° C., they can be directly applied to conductive backings such as metal foils and heated together with the conductive backings for crosslinking the conductive pressure-sensitive adhesive layers, thereby yielding conductive tape having conductive pressure-sensitive adhesive borne thereon. This suggests that these prior art conductive pressure-sensitive adhesives have a serviceable temperature range from 80° C. to 120° C. at the highest. They cannot be used at higher temperatures beyond the range because the pressure-sensitive adhesive layer will soften and substantially lose bond strength.
When it is desired to join a cable to a metal which is unsusceptible to soldering, one approach is by attaching a metal foil tape having conductive pressure-sensitive adhesive borne thereon to the surface of the metal and soldering the cable to the metal foil tape for grounding or similar purposes. Prior art pressure-sensitive adhesives cannot withstand the high temperature of soldering. If the adhesive layer of the tape softens, there is a likelihood that the tape will be shifted, the metal member to which the tape is attached fall off, or the tape as a whole be separated, thus failing to attain the grounding or similar purposes. Recently, rubber connectors featuring high conductivity and heat resistance are in increasing use. It is desired to have a heat resistant, conductive pressure-sensitive adhesive which is applicable to such rubber connectors.
To overcome the foregoing problem, an attempt has been made to employ heat resistant resins such as polysiloxane as the pressure-sensitive adhesive. Known silicone pressure-sensitive adhesives include those comprising a polysiloxane consisting essentially of Me3SiO1/2 units and SiO4/2 units, known as MQ resin, and a dimethylsilicone gum, or a condensed product thereof. These adhesives are generally crosslinked with peroxides at high temperatures of 150 to 180° C. in order to enhance the bond strength or cohesion. JP-A 63-22886 discloses a low-temperature curable addition type silicone pressure-sensitive adhesive comprising an alkenyl-containing organopolysiloxane, MQ resin and Si—H radical-containing polysiloxane.
These silicone base pressure-sensitive adhesives are admixed with a metal powder as a conductive powder to formulate conductive pressure-sensitive adhesive compositions, which have the drawback that they fail to develop satisfactory conductivity unless large amounts of conductive powder having a high specific gravity are blended. Due to the difference in specific gravity, the conductive powder tends to separate from the silicone component so that the composition is inevitably poorly conductive, too viscous and inefficient to work. In this connection, JP-A 1-287169 proposes to dilute such a composition with an organic solvent to provide a silicone base pressure-sensitive adhesive composition which is reduced in viscosity buildup and efficient to work. However, the problem of unstable conductivity due to settlement and irregular dispersion of conductive powder has not been solved.
The conductive pressure-sensitive adhesive compositions obtained by admixing silicone base pressure-sensitive adhesives with a metal powder as a conductive powder suffer from problems associated with the conductive powder for imparting and developing conductivity. For instance, although silicone resins are resistant to heat at or above 200° C. in air, the conductive properties of pressure-sensitive adhesives at such temperatures apparently decline with the passage of time. That is, although silicone resins are stable in an environmental temperature region of at least 200° C., the adhesives fail to exhibit stable conductivity in the same temperature region. This is because the conductive powder used is a copper, nickel or similar metal powder which is susceptible to oxidation by heat. When exposed to a high temperature environment, the conductive powder in the form of a metal powder is oxidized by heat to invite the detrimental effect that the powder discolors or loses conductivity. When conductive pressure-sensitive adhesives are used for fixedly securing conductive silicone rubber connectors for high-reliability electronic materials, there is a desire to stabilize the conductivity of the adhesives when held at such high temperatures.
On the other hand, Japanese Patent No. 2,928,944 and JP-A 2001-146578 disclose a metal powder and a metal coated powder as conductive powders, respectively. Noble metals such as gold and platinum used as the conductive powder have both heat resistance and conductivity, but are very expensive and have a high specific gravity which gives rise to a problem of settlement over time. Thus noble metals are not commercially used in practice. Instead, silver is often used in the application where very high conductivity is required, and copper or nickel used in other applications. Copper and nickel powders are inexpensive, but when used under heated conditions, tend to form an oxide film on surfaces of particles under the influence of air-borne oxygen and moisture, detracting from conductivity. Then the silver powder which is highly conductive and less susceptible to oxidation or a metal powder in the form of copper particles surface coated with silver is often used as the conductive powder. The silver powder is still unsatisfactory because of its drawback that particles settle over time due to the high specific gravity.
For the metal-coated powder, a compromise between a low specific gravity and conductivity can be found by selecting a low specific gravity powder as core particles. However, the metal-coated powder has the problem that when exposed to high temperatures, the metal coating is stripped from the core particles on account of remarkably poor adhesion therebetween. This prohibits the use of the metal-coated powder in conductive pressure-sensitive adhesives for electronic materials requiring high reliability.
The present inventors disclosed in JP-A 2002-167512 that a conductive powder in which core particles of an inorganic material or organic resin are surface treated with a silicon-base polymer having reductive effect and over-coated with a metal by electroless plating can be used in silicone rubber requiring high reliability because the adhesion between the core particles and the metal coating is improved to a sufficient extent to eliminate the problem of the metal coating being stripped from the core particles upon exposure to high temperatures. This conductive powder has never been used in pressure-sensitive adhesives.